I think you have structured your letter very well.
The only thing I would change is - I would not apologise for the length of your letter & the detail of it. You have nothing to apologise for - had they not been decieved by the breeder or 'dealt' with her in the way they did - you nor Lucy would be in this situtation
I really hope they take on board your comments & do refund you your vet fees. Keep us posted.